This invention relates to probes for sensing the position of a workpiece, e.g. by contact, for use in apparatus such as co-ordinate measuring machines and machine tools, for measuring the dimensions of work-pieces. More particularly, it relates to circuits for processing and conditioning the signals received from such probes.
Our European Patent Application No. 87105192.6, U.S. Pat. Application Ser. No. 07/036,475 and Japanese Patent Application No. 91,112/1987, which are co-pending with the present application and claim a common priority therewith, and which are incorporated herein by reference, describe a probe having a stylus for contacting the workpiece. The probe is fitted in apparatus which causes relative movement of the probe and workpiece, and the probe contains two different means for sensing when such contact is made. One sensing means is a piezo-electric sensor, which detects a sharp acceleration or shock transmitted into the stylus on its initial contact with the workpiece, and gives a practically instantaneous indication of this contact. The other sensing means relies on conventional electro-mechanical switching between a movable member to which the stylus is connected, and a fixed member or housing. This switching occurs between confronting seat elements of a kinematic support for the movable member on the fixed member, which may comprise a cylinder seating on a pair of balls at each of three locations spaced around the axis of the stylus. The purpose of this switching is to indicate seating and reseating of the movable member. It also provides a useful back-up signal to indicate contact with the workpiece if the piezo-electric sensor fails to provide a signal on initial contact (e.g. if the contact is made at a very low speed). This back-up signal provides a (less accurate) measurement indication, and also serves to stop the apparatus to prevent collision damage between the workpiece and the probe.
The two signals from the two sensing means are combined to give a single trigger signal, which is taken to the apparatus to indicate that contact has occurred. The apparatus therefore only has to deal with one trigger signal, as with a conventional probe. To give maximum interchangeability between conventional probes and such probes with two sensing means, the above co-pending applications describe circuits for combining the two signals within the probe itself, prior to transmission to an external interface unit which interfaces the probe to the apparatus. However, it would of course be possible to combine the signals in the interface unit if maximum interchangeability is not essential.
However, while it is advantageous to provide the apparatus with a single combined trigger signal in the above manner, the apparatus has no way of knowing whether the trigger signal resulted from the accurate sensing of the initial contact provided by the piezo-electric sensor, or from the less accurate switching sensor.